Anoxia-Induced Changes in Extracellular K⁺ and pH in Mammalian Central White Matter

In gray matter (GM), anoxia induces prominent extracellular ionic changes that are important in understanding the pathophysiology of this insult. White matter (WM) is also injured by anoxia but the accompanying changes in extracellular ions have not been studied. To provide such information, the time course and magnitude of anoxia-induced changes in extracellular K⁺ concentration ([K⁺]_o) and extracellular pH (pH_o) were measured in the isolated rat optic nerve, a representative central WM tract, using ion-selective microelectrodes. Anoxia produced less extreme changes in [K⁺]_o and pH_o in WM than are known to occur in GM; in WM during anoxia, the average maximum [K⁺]_o was 14 ± 2.9 mM (bath [K⁺]_o = 3 mM) and the average maximum acid shift was 0.31 ± 0.07 pH unit. The extracellular space volume rapidly decreased by ∼20% during anoxia. Excitability of the rat optic nerve, monitored as the amplitude of the supramaximal compound action potential, was lost in close temporal association with the increase in [K⁺]_o. Increasing the bath glucose concentration from 10 to 20 mM resulted in a much larger acid shift during anoxia (0.58 ± 0.08 pH unit) and a smaller average increase in [K⁺]_o (9.2 ± 2.6 mM). The increased extracellular glucose concentration presumably provided more substrate for anaerobic metabolism, resulting in more extracellular lactate accumulation (although not directly measured) and a greater acid shift. Enhanced anaerobic metabolism during anoxia would provide energy for operation of ion pumps, including the sodium pump, that would result in smaller changes in [K⁺]_o. These effects were probably responsible for the observation that the optic nerve showed significantly less damage after 60 min of anoxia in the presence of 20 mM glucose compared to 10 mM glucose. Under normoxic conditions, increasing bath K⁺ concentration to 30 mM (i.e., well beyond the level shown to occur with anoxia) for 60 min caused abrupt loss of excitability during the period of application but minimal change in the amplitude of the compound action potential following the period of exposure. The anoxia-induced increase in [K⁺]_o, therefore, was not itself directly responsible for irreversible loss of optic nerve function. These observations indicate that major qualitative differences exist between mammalian GM and WM with regard to anoxia-induced extracellular ionic changes.